The long term objective of this research project is to formulate a set of rules that will predict both the type and probability of specific chemical alterations produced in chromatin DNA by the direct effects of ionizing radiation. Achievement of this objective is important to defining the risks associated with long term exposures to low doses of radiation: risks such as the induction of cancer or leukemia. The specific aim is to fully characterize the free radical processes by which ionizing radiation, through direct effects, alters the chemical structure of the nucleic acid bases. A full characterization entails 1. the identification of the pristine free radicals, 2. the monitoring of the subsequent reactions, 3. the measurement of the yield and destruction constants, and 4. the determination of the influence of the host matrix on radical formation, trapping, and subsequent reactions. The bases will be studied in the form of single crystals using electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) spectroscopy. The free radical products and reactions will be studied over a temperature range of 4.2K to 300K. The series of crystals scheduled for study consist of simple derivatives of the common nucleic acid bases. An emphasis will be given to thymine and adenine derivatives as the work is targeted toward working out the distribution of free radical damage in a 1-methylthymine:adenosine co-crystalline complex.